1. Technical Field
The present invention relates in general to an improved structural panel, and in particular to an improved structural panel with an undulated surface. Still more particularly, the present invention relates to an undulated structural panel concurrently formed from composite materials and syntactic or foamed resins.
2. Description of the Prior Art
In the prior art, structural panels are typically stiffened by attaching individual stiffening elements to the panels, such as hats, blades, xe2x80x9cJ""sxe2x80x9d, or xe2x80x9cC""sxe2x80x9d. These additional elements increase the cost of fabricating the panels and are limited in adaptability. Although stiffened structural panels have also been achieved by incorporating honeycomb core or similar materials between two laminate faces having the appropriate planar or smoothly curved profile, concurrent stiffening by this latter method has limitations on panel shape and can be costly to practice with control.
Similarly, stiffening via a geometry of corrugations or beads has not been practical. Simple, low-cost fabrication of composite structures with beaded or corrugated stiffening elements has been difficult because inextensible fibers cause bridging or wrinkling. This problem is often present even when laid layer by layer to the contour of the forming tool. Consequently, the corrugations or beads of such a structure cannot be readily stiffened in a direction normal to the corrugations by incorporating a layer of syntactic or foamed resins to create a sandwich effect.
An uncured, thermoset resin sheet is reinforced with oriented fibers and is slit to define a desired length or lengths for the fibers. A series of the sheets is cut and stacked to form integrated layers of the composite material for a structural panel. The panel has two such outer layers that sandwich two shorter layers on each end, and a syntactic or foamed resin layer in between. The two shorter layers are required for the condition where it is desirable to isolate and protect the syntactic resin from direct exposure to humidity or for some other such purpose. In many cases, the shorter layers may be omitted, leaving the syntactic resin visible at the ends. Each of the composite layers is formed from the same materials and by the same process, but may vary in the directional orientation of their fibers.
The uncured panel is assembled into a flexible, substantially flat configuration and then is heated and formed to the contours of a tool having an undulated surface geometry. The panel is further heated to cure the combined composite and syntactic resins into a series of rigid undulations that permeate each of its layers. In one embodiment, the undulations are in a smooth, sine wavelike pattern that allow the panel to maintain a planar configuration. Although the fibers themselves do not stretch, the short lengths of the fibers enable stretching of the material in the fiber direction so that deformation of the composite is possible in all directions. The panel is stiffened in one direction by spacing its outer layers apart with the syntactic layer, and in an orthogonal direction by its undulated surface.
Accordingly, it is an object of the present invention to provide an improved structural panel.
It is an additional object of the present invention to provide an improved structural panel with an undulated surface.
Another object of the present invention is to provide an undulated structural panel that is formed in one operation from a flat or appropriately contoured composite laminate.
Still another object of the present invention is to provide an undulated structural panel concurrently formed from composite materials and syntactic or foamed resins.